1. Field of the Invention
The present invention relates to a wireless power supply technique, and particularly to a foreign object detection technique.
2. Description of the Related Art
In recent years, in order to supply electric power to an electronic device, contactless power transmission (which is also referred to as “contactless power supply” or “wireless power supply”) has begun to come into commonplace use. In order to advance the compatibility of products between manufacturers, the WPC (Wireless Power Consortium) has been organized, and the WPC has developed the Qi standard as an international standard.
The wireless power supply that conforms to the Qi standard uses electromagnetic induction between a transmission coil and a reception coil. A power supply system is configured including a power supply apparatus having a power transmission coil and a power receiver terminal having a reception coil.
FIG. 1 is a diagram showing a configuration of a wireless power supply system 10 that conforms to the Qi standard. The power supply system 10 includes a power transmission apparatus 20 (TX: power transmitter) and a power receiving apparatus 30 (RX: power receiver). The power receiving apparatus 30 is mounted on an electronic device, examples of which include cellular phone terminals, smartphones, audio players, game machines, and tablet terminals.
The power transmission apparatus 20 includes a transmission coil (primary coil) 22, a driver 24, a controller 26, and a demodulator 28. The driver 24 includes an H-bridge circuit (full-bridge circuit) or otherwise a half-bridge circuit. The driver 24 applies a driving signal S1, specifically, in the form of a pulse signal, to the transmission coil 22. This provides a driving current flowing through the transmission coil 22, which generates an electric power signal S2 at the transmission coil 22 in the form of an electromagnetic signal. The controller 26 integrally controls the overall operation of the power transmission apparatus 20. Specifically, the controller 26 controls the switching frequency of the driver 24 or otherwise the duty ratio of the switching of the driver 24 so as to adjust the electric power to be transmitted.
In the Qi standard, a protocol is defined for communication between the power transmission apparatus 20 and the power receiving apparatus 30, which enables information transmission from the power receiving apparatus 30 to the power transmission apparatus 20 via a control signal S3. The control signal S3 is transmitted from a reception coil 32 (secondary coil) to the transmission coil 22 in the form of an AM (Amplitude Modulation) modulated signal using backscatter modulation. The control signal S3 includes electric power control data (which will also be referred to as a “packet”) which controls an amount of electric power to be supplied to the power receiving apparatus 30, and data which indicates the particular information for identifying the power receiving apparatus 30. The demodulator 28 demodulates the control signal S3 included in the current or otherwise the voltage applied to the transmission coil 22. The controller 26 controls the driver 24 based on the power control data included in the control signal S3 thus demodulated.
The power receiving apparatus 30 includes the reception coil 32, a rectifier circuit 34, a smoothing capacitor 36, a modulator 38, a load 40, a controller 42, and a power supply circuit 44. The reception coil 32 receives the electric power signal S2 from the transmission coil 22, and transmits the control signal S3 to the transmission coil 22. The rectifier circuit 34 and the smoothing capacitor 36 rectify and smooth a current S4 induced at the reception coil 32 according to the electric power signal S2, thereby converting the current S4 into a DC voltage.
Using electric power supplied from the power transmission apparatus 20, the power supply circuit 44 charges an unshown secondary battery or steps up or otherwise steps down the DC voltage VRECT, so as to supply the DC voltage to the controller 42 or other circuits such as the load 40.
The controller 42 monitors the amount of electric power supplied to the load 40, and accordingly generates electric power control data which controls the electric power to be transmitted from the power transmission apparatus 20. The modulator 38 modulates the control signal S3 including the electric power control data so as to modulate the coil current that flows through the reception coil 32, thereby modulating the coil current and coil voltage applied to the transmission coil 22.
With the power supply system 10, the power transmission apparatus 20 and a power receiver terminal (electronic device) are located within a space with a relatively high degree of freedom. Thus, it is conceivable that a state can occur in which there is an electro-conductive foreign object such as a piece of metal or the like between, or otherwise in the vicinity of, the transmission coil 22 and the reception coil 32. In a case in which wireless power supply is performed in such a state, a current flows through such a foreign object, leading to a problem of power loss. Also, this leads to a problem of the foreign object heating up. In view of such a situation, foreign object detection (FOD) has been designed in the WPC1.1 (System Description Wireless Power Transfer Volume I: Low Power Part 1: Interface Definition Version 1.1) specification.
In such FOD, the electric power transmitted by the power transmission apparatus 20 is compared with the electric power received by the power receiving apparatus 30. When the difference between them is greater than an allowable value, judgment is made that an abnormal state is present.